Compositions of matter containing ferromagnetic particles and nonferromagnetic aluminum particles in an elastic material

ABSTRACT

COMPOSITIONS OF MATTER COMPRISING ALUMINUM PARTICLES, IRON OR OTHER FERROMAGNETIC PARTICLES AND AN ELASTIC BINDER FOR CEMENTING THE PARTICLES INTO A COHERENT MASS.

0 1973 a s. v. R. MASTRANGELO 3,766,095

COMPOSITIONS OF MATTER CONTAINING FERROMAGNETIC, PARTICLES ANDNONFERROMAGNETIC ALUMINUM PARTICLES IN AN ELASTIC MATERIAL Filed Nov. 2,1971 FIG-1 FIG-Z INVENTOR SEBASTIAN V. R. MASTRANGELO ATTORNEY UnitedStates Patent O ABSTRACT OF THE DISCLOSURE Compositions of mattercomprising aluminum particles, iron or other ferromagnetic particles andan elastic binder for cementing the particles into a coherent mass.

BACKGROUND OF THE INVENTION (1) Field of the invention (2) Descriptionof the prior art Sawyers, McCarthy and Jacoby, in Technical MemorandumSCTM 293-60-52, Sandia Corp., Livermore, Calif. (1960), report thataluminum powder of commercial grade and known to contain magnetic metalin appreciable quantities exhibits switching properties when closelybound as finely divided material particles in a homogeneous mass bymeans of dielectric filler. Other than a suggestion that the magneticmaterial is probably iron, the identity, amount and purpose or efiect,if any, of the magnetic material are not disclosed.

Gibbons and Beadle disclose in Solid State Electronics, Pergamon Press1964, vol. 7, pp. 785-797 that films of nickel oxide, films of othermetallic oxides, anodized aluminum, and aluminum powder held in asuitable insulating binderas in Sawyers above, all have similarelectrical switching properties. They list these properties of a typicalswitch operating between high and low resistance states referred to asOFF- and ON-states:

(1) It has an OFF-state in which its resistance is approximately 25megohms.

(2) It has an ON-state in which its resistance is approx imately 100ohms.

(3) The device may be turned from its OFF-state to its ON-state byapplication of a two-hundred volt pulse of 40 microseconds duration. Thepulse source impedance should be high (approximately 100 thousand ohms).

(4,) The device may be turned from its ON-state to its OFF-state byapplication of a 150 milliamp current pulse, 10 nanoseconds in duration.

(5) Devices made to date have a limited lifetime. The

, maximum number of repetitive switching cycles obtained thus far is1000. Furthermore, the device fails short; that is, it cannot beswitched out of its ON- state with normal current amplitudes.

Two problems which commonly arise, therefore, in switching devicesformed fiom aluminum and/or other materials in an insulating binder are:(1) high currents must normally flow during current pulsing in order toswitch to the OFF-stateand (2) the maximum number of repetitiveswitching cycles is limited to about 1000 before the device fails shortor burn-ON.

New compositions of matter have now been discovered which, whencontacted with electrodes and activated by an electrical voltage pulseto a state of lower resistance, serves as a useful electrical switchingdevice which minimizes these problems. Switching devices formed asdescribed herein normally require only 0.1 to 10 milliamp current pulsesto be turned from ON-state to OFF-state, and can be cycled about 10times or more without failure.

SUMMARY OF THE INVENTION This invention is directed to compositions ofmatter comprising (a) aluminum particles, (b) iron or otherferromagnetic particles, and (c) an elastic binder for said particles,wherein the ratio of ferromagnetic to aluminum particles by weight is inthe range of 1:6 to about 2:1 and the combined weights of the aluminumparticles and the ferromagnetic particles is from about 40 to about ofthe total weight of said particles and the elastic binder.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an activating circuit diagram.FIG. 2 is a diagram of a pulse circuit for repetitive cycling.

DESCRIPTION OF THE INVENTION It has been discovered that theincorporation of a ferromagnetic component in controlled amounts into adispersion of an aluminum powder in an elastic binder greatly improvesthe performance of the resulting compositions when they are contactedwith electrodes and activated by an electrical voltage pulse suppliedthrough said electrodes to a state in which it can serve as a bistableswitch.

Better performance is evident by direct comparison of switches orswitching devices prepared from the compositions of this invention withthe performance of those made without the addition of the specifiedamount of ferromagnetic powder. By comparison, after activation, theswitches prepared from the compositions of this invention, as a resultof the presence of the ferromagnetic component, turn OFF and stay OFFwhen they are switched OFF by a current-limited pulse and require lesscurrent to effect the transition. In life tests they do not burn-ON asreadily. These advantages and others are attained by utilizingferromagnetic particles in the compositions of the present invention.

Thus, a switching device made from the compositions of this inventioncan exist in any of three different states, a latent state, an ON-state,and an OFF-state. In the latent state, the resistance of the device istypically greater than 10 ohms; similarly, the resistance in theOFF-state is typically of the order of 10 ohms. However, in theON-state, the resistance is typically only from 10 to 2.5 10 ohms, atleast 10 times less than the latent or OFF-state.

As originally formed, a switching device of this in vention is in thelatent state. In a manner described below, it can be altered from thelatent state to the ON-state.

The transition from the latent state to the ON-state is calledactivation and is accomplished by applying What is called the activatingvoltage which is equal to or greater than a critical threshold orbreakdown voltage. Typically the activating voltage is between and 400volts per centimeter and it is applied as a brief pulse.

If the device is in the ON-state, it can be switched to the OFF-state,i.e., turned-OFF, by application of a small current-limited pulse ofabout 0.1 to 25 milliamperes, less than 10 milliamperes being preferred,and regulated such that at the end of the pulse the current drops veryrapidly to a very low value. By a current-limited pulse is meant vIf thedevice is in the OFF-state, it can be switched to the ON-state byapplication of a voltage, i.e., turn-N voltage, typically between aboutvolts and 225 volts and regulated such that at the end of the pulse thecurrent drops comparatively slowly to a low value. By burn-ON is meantthe transition to a state such that the device cannot be switched out ofits ON-state. By a bistable switch is meant a switch that has tworesistance states, an ON-state, and an OFF-state. Such a switch exhibitspractically no other discrete resistance values in transit between theO'N- and OFF-states, as can be established by viewing on an oscilloscopescreen the voltage across a small, fixed series resistor in response toan applied voltage pulse.

The novel compositions of this invention comprise:

(1)- non-magnetic metal particles of aluminum,

(2) ferromagnetic particles present in an amount such that the ratio offerromagnetic to aluminum particles by weight is in the range of 1:6 toabout 2:1, and

(3) an elastic binder for said particles.

Ferromagnetic metal powders useful in the compositions of this inventioncan be characterized according to methods described by Lark-Horovitz andJohnson, Solid State Physics, vol. 6, Part B, p. 204 (1959) AcademicPress, as experiencing a measurable body force of a few milligrams in animposed magnetic field. In general, the ferromagnetic material will havea saturation magnetization per unit volume of at least about 100,preferably at least 500 cgs. units of magnetic moment per unit volume.Ferromagnetic materials useful in this invention include saturationmagnetizations 1752, 1446, and 512, respectively. Iron is preferred inmaking the compositions of this invention. 1

The ferromagnetic powder is selected to have an average particle size of10 to 30 microns and preferably a narrow distribution of sizes aroundmicrons.

Ferromagnetic alloys having high saturation magnetization are alsoavailable in the desired powder form and particle size range and may beuseful in this invention. Such alloys may contain other elements of thePeriodic Table in addition to one or more of the three cited.

The aluminum metal powder component preferably meets the followinggeneral criteria: (1) an average particle size of from about 10 to 30microns, more preferably a narrow distribution of sizes around 20microns, (2) atomized powders that are granular in shape.

By the term elastic binder is meant an insulating material which iscapable of elongation with substantial recovery of its originaldimensions.

Preferably, the elastic binder (when tested without the aluminum andferromagnetic particles) should be capable of being elongated at least100% (A.S.T.M. D412 test), and still retract to less than 1.5 times itsoriginal length.

The elastic binder should be present in an amount such that the combinedweights of the non-magnetic aluminum particles and the ferromagneticparticles comprise from 40 to 85 percent of the weight of said particlesand the elastic binder. The elastic binder may be dissolved in asuitable carrier solvent and the aluminum and ferromagnetic particlesadded thereto to form a dope.

The nature of the elastic binder itself can vary widely and itscomposition is not critical provided it is sufiiciently elastic asdefined. Binders with such elastic properties include natural rubber,synthetic polyisoprene rubber, elastomeric chloroprene polymers,fluorolefin elastomers, butadiene-styrene rubber,ethylene-propylene-nonconjugated diene rubbers, silicone rubbers andrubbery condensation polymers such as polyurethanes obtained by reactionof polyisocyanates with polyalkylene glycols. The elastic binder mayalso contain fillers, reinforcing agents or plasticizers commonly addedto elastomers, providing the properties of the resultant binder remainwithin the limitations hereinbefore recited.

Still polymers with rigid molecular structure such as aromaticpolyamides, polyimides and polystyrene result in switches that do notswitch off. The elongations of such binders are about 60%, 8%, and25-58%, respectively, all less than the specified lower limit ofelongation.

For convenience in fabricating switches, by casting flexible sheets forexample on which many switches can be formed side by side, it isdesirable to handle fluid or fluidizable compositions from which thefinal switch composition can be formed in place. Accordingly, instead ofthe normally solid elastic binder by itself or in a solvent, there canbe employed an elastic binder-forming material along with the aluminumand magnetic powder components.

Such elastic binder-forming material includes any one or more of thefollowing:

(1) preformed polymer which can be further cured to an elastic binder, acuring agent, and optionally a carrier solvent as above,

(2) preformed polymer and optionally a carrier solvent,

said polymer being curable by heat or irradiation,

(3) polymer precursor, chemical agent to convert said precursor intoelastic binder, and optionally an inert volatile solvent as thinner,

(4) liquid prepolymer, self-curing or containing a curing agent.

By carrier solvent as used herein is meant a liquid dispersion mediumfor transporting one or more substances, such as the particles of thisinvention, which also is capable of solubilizing other materials such ascuring agent or chemical agent for polymerization if such be present,e.g., acetone, xylene, tetrahydrofuran, benzene, toluene,dimethylacetamide, ethyl ether, chloroform and dimethylformamide. Saidcarrier solvent need not be completely removed by subsequent treatmentprovided the required criteria for elongation and recovery are met bythe resultant binder.

In making compositions of matter which lie within the spirit of thisinvention, dopes may therefore be used which are dispersions of aluminumand ferromagnetic particles in polymer solutions in volatile carriersolvents as mentioned above, e.g., a solution of hydrocarbon rubber inbenzene or toluene. Another type of dope might also contain a reactantin addition to the solvent to promote further polymerization of anelastic binder forming mate rial that either may or may not yet besufiiciently elastic to meet the required criteria for elongation andrecovery; for example, a dope useful in making switches contains 20 wt.percent polyurethane rubber such as Adiprene C, a reaction product ofdiisocyanate and polyalkylene ether glycol in dimethylformamidecontaining 3.5 v./v. percent H O whereas other useful formulationsinclude blends of powders in self-curing liquid prepolymers such assilicone rubbers. If desired, elastomers capable of undergoing furtherreaction, such as chain extension or crosslinking, to harden but stillkeep products elastic, can be cured in situ (in the presence of themetal components). For example, curing agents such as peroxides orsulfur for unsaturated systems represented by hydrocarbon rubbers(including natural and synthetic rubbers derived from olefins andpolyolefins) can be incorporated into the compositions of this inventionand subjected to curing conditions that are well known, for example,curing by heating. Alternatively, rubbers can be cured by irradiationunder conditions known to the art for hardening them.

The order in which the components of the compositions of this inventionare mixed is not critical nor is the temperature and pressure at whichthe mixing takes place. Normally the ferromagnetic metal powder and thealumlnum metal powder are first mixed together. Gentle mixing in atumbler mixer is preferred to preserve the natural protective tarnishfilm of aluminum oxide which imparts a characteristically dull graycolor to aluminum metal particles handled in air. The mixedferromagnetic and aluminum metal powders are then blended with theelastic binder. Another satisfactory approach is to blend the powdersseparately, first one then the other, with elastic binder to form thecompositions of this invention, the order of addition again not beingcritical.

According to this invention, the combined weights of the aluminumparticles and the ferromagnetic particles comprise from about 40 to 85percent of the total weight of said particles and the elastic binder.Compositions containing more than 85 percent generally containinsuflicient binder for mechanical strength. Percentages of 60-70% arepreferred. Compositions containing less than 40 percent of the combinedweight frequently burn-ON.

The ratio of the weight of the ferromagnetic powder to the weight of thealuminum powder must be in the range of about 1:6 to 2:1. The upperlimit of 2:1 must be observed in order to avoid the burning-ON ofswitches, but not so closely at low combined particle weights near 40%as at high loadings near 85%. The preferred ratio is about 1.5: 1. Atthe lower limit of 1:6 switches are on the verge of failing to switchOFF, the current required to switch OFF approaches but is still lessthan the high current required without the ferromagnetic componentpresent, and the switches at this ratio also tend to conduct alongmultiple paths between electrodes rather than a single path. Instead ofacting as a bistable element such switches tend to develop and spendtime in states of intermediate electrical resistance between the ON- andOFF-states upon being pulsed. Multiple paths can be detected bydisplaying the switch current, e.g., by taking a voltage signal from afixed resistor in series with a switch, and displaying it on the screenof an oscilloscope as the switch is cycled between ON- and OFF-states byalternate application of current-limited and voltage pulses. If multiplepaths exist, additional horizontal lines or steps will appear betweenthe two widely separated horizontal lines or steps characteristic of theON- and OFF-states during each switching cycle. Occasionally one or evenseveral such lines of faint brightness may be seen indicative of atendency to conduct along one or more conductive paths other than theactivated path of lowest resistance. If all the intermediate lines arevery faint, operation of a switch as a bistable element is usually notimpaired; however, when one such line approaches the brightness ofeither the ON- or OFF-state, or becomes as bright as the OFF-state line,bistable switching may become uncontrollable.

As formed by solvent evaporation, melt techniques, or polymerizationprocedures, the compositions described herein typically have electricalresistivities greater than or ten billion ohm-centimeters beforeactivation.

A switching device made from the compositions of this invention may beformed from a dope by shaping the dope, rendering it form-stable, andthen applying two noncontiguous electrodes. The dope may be shaped byspreading it onto a substrate on which it remains when in use or fromwhich it is removed before use. It may be spread onto the selectedsubstrate by brushing, dipping, pouring use of a doctor-knife, andsimilar procedures. After the dope has been shaped, it is subjected toheat and/or vacuum to render it form-stable, that is, to remove volatilesolvent and bring the properties of the elastic binder into the rangehereinbefore recited.

Coated wires are made by using a wire as a substrate and dipping it intothe dope. Either before or after the dope has been rendered form-stable,additional electrode or electrodes are placed in contact with it. Thewire serves as one electrode, and each combination of the wire, switchmaterial, and additional electrode serves as a switching device.

Fibers may be pulled from the dope of this invention. Either before orafter being rendered form-stable, such fiber can be used to form aswitch device by being cemented to two electrodes by the dope of thisinvention or any conductive cementing material.

Fiber bridges with a common terminal are made into switch arrays withone contact serving as a common ter-.

minal for several switches.

A preferred composition of this invention is made by mill-blending,e.g., in a two roll rubber mill, equal weights of aluminum powder, ironpowder, and an elastic terpolymer of ethylene, propylene and anunconjugated diene, e.g., 1,4-hexadiene with sufiicient dicumylperoxideby weight of the terpolymer to effect curing. This and other similarterpolymers having suitable elastic properties are disclosed in US.2,933,480. After'milling, the blend is hot-pressed into sheets which arecured. In the form of such sheets many applications in the computer orelectronic fields are accommodated. For example, electrodes and printedcircuitry may be formed on such sheets for use as read only memories andread-write memories. The sheets is easily cut into any sized or shapedsmaller pieces for use as electronic circuit components in flip-flops oroscillators. Electrical contact with the sheet is made with paintedelectrodes or with suitable spring contact probes.

Glass, metal, plaster, rubber, wood and paper are satisfactorysubstrates for the compositions and dopes of this invention; preferencesare for polyester film or no substrate at all.

Variation of the switch sheet or switch plate form includes ametal-backed switch plate made by casting a film of switch dope on asheet of aluminum foil. The film dries to a reduced thickness andopposing spring contacts are aflixed. Other variations include a paperreinforced sheet made by padding various switch dopes on tissue paper, aplastic-backed switch sheet made by casting various switch dopes on apressure-sensitive Mylar polyester film, and coated printed circuitboards, made by casting various switch dopes on printed circuit boardswith or without printed circuits in place.

In order to make a useful bistable switch from a latent switch of thecompositions of this invention between two electrodes, a voltage pulsemust be applied to the switch composition to form a conductive path ofless than one megohm resistance per centimeter. By application of suchan activating voltage pulse specific resistance values of the initialON-state can be attained ranging from Ohms to 250,000 ohms percentimeter. Once a conductive path has been established its resistanceremains essentially unchanged during identification of the ON-state byany small testing or reading voltage not exceeding a voltage potentialwhich produces enough current to cause a transition to the OFF-state,e.g., less than about 5 volts per centimeter.

The electrical resistance of the initial ON-state depends on themagnitude of the activating voltage as well as the nature, particlesize, and amount of dispersed particles. In general the initialresistance is decreased by increasing the activating voltage above acritical threshold level for activation or by using larger particles. Itcan, however, also be decreased by reducing the size of a seriesresistor, nominally maintained at 330,000 ohms, which is used to limitthe current which flows when the activating voltage pulse is applied.When the series resistor is reduced in size the rate of decay of theactivating pulse may become so rapid that the switching device is notonly activated to the ON-state to become a useful switch, but within theduration of the pulse passes through the ON-state and is left in theOFF-state at the end of the pulse. The reason for this will be betterunderstood following a subsequent description of the nature ofcurrent-limited pulses required to turn off switches. Switches activatedin this manner are as useful as those activated to an ON-state providedthe switch is not impaired by an excessive surge of current. Thus, aswitch with desired electrical properties within those practical withthe materials used can be obtained from any variety of combinations ofactivating voltage, current, and particle size and amount ofnon-ferromagnetic aluminum and ferromagnetic powders.

Two terminal electrodes are needed to apply the activating voltagepulse. Electrode shape, size and form make little difference in switchperformance. Silver, copper, and gold paints, copper wire (30 gauge and18 gauge) straight pins, pressure-sensitive-backed metal foils, roundedspring-loaded pressure contacts and alligator clips have all been usedsuccessfully.

Between the two terminal electrodes standing oppositely across a 0.5 cm.sample path for example, a difference in electric potential or voltageof 150 to 400 volts is normally required to activate the switch. Highervoltages tend to produce ON-states of lower resistance but applicationof too high a voltage results in switches that will not turn- OFF. In apreferred manner a resistance of less than 250,000 ohms is attained byapplying a voltage pulse which is limited so as to be nearly equal tothe threshold voltage of the switch and relatively independent ofvariations in switch-forming compositions. Attempted activation withless than the threshold voltage may have deleterious effects.Incompleted paths may form which may in turn produce multiple paths whenbreakdown is finally reached or during switching operation.

The latent switches prepared from the compositions of this inventionshould therefore be activated by circuitry that will standardize andproduce uniformity in switch characteristics and performance. A typicalcircuit for activation of a switching device, prepared from thecompositions of this invention, from its latent state to its ON- stateis shown in FIG. 1. This circuit is optimized for a switch with about 1cm. spacing between electrodes. An initially open single-pole doublethrow switch 1 is thrown to terminal 2, allowing a source of electricpotential 3 of 400 volts strength to energize a 0.001 ,uf. capacitor 4.Switch 1 is then thrown to make connection with terminal 5, whereuponthe potential difference across latent switching device 6 rises at arapid but controlled rate until its activation occurs.

Two means of control are provided. A parallel circuit path consisting ofa 270,000 ohm resistor 7 provides a finite time constant for dischargeof the energizing capacitor 4 since the latent switching device has toohigh a resistance to do so, typically ohms, before activation. Secondly,a 100 ,u/Lf. time delay capacitor 8 serves to slow down the rate of riseby receiving electrical charge flow from the energizing capacitor 4. Thecapacitor 8 establishes a time constant for potential difference risedetermined by the product of the value of the adjacent 10,000 ohmresistor 9 and its own capacitance in farads equal to one microsecond.Within this order of time a threshold voltage between about 150 and 400volts is thereby reached for the activation of the latent switchingdevice, changing its electrical resistance from a high value typical ofits latent state to a low value characteristic of its ON- state.Thereafter a 300,000 ohm resistor 10in series with the device limits theresultant increase in current through it as the potential differencethat persists is rapidly dissipated. Finally, a 1N-4005 silicon diode 11shorts out any reverse transient voltages that might develop.

If the potential difference is allowed to rise too rapidly, the voltagevalue may overshoot the threshold or breakdown voltage and produce aswitch that will not turn- OFF.

If the current is not limited when the switching device is activated, itwill tend to pass through the ON-state and be turned-OFF by the currentsurge. Sometimes actual destruction of the device will occur.

If the diode is omitted, the reverse transient voltages are capable ofsometimes destroying the device.

By observing these criteria for composition preparation and switchactivation, the current to switch OFF is greatly reduced and a degree ofreliability is achieved which has been missing in switches made withoutthe addition of a ferromagnetic component. The current needed'to'turn-OFFthe switchesis normallyE0;1 to :10 milliamperes and mostfrequently from 1.to 5 milliamperes, instead of 10 to 200 milliamperescharacteristic of switches made without the addition of ferromagneticpowder. An ordinary switching circuit will not,however, sufiice unlessit provides rapid decay of the trailing edge of the turn OFF pulse. Thisis evident, for instance, because an activated switching device will notturn-OFF in response to a 60 Hz. or even a 1000 Hz. pulse form.

A typical circuit useful for turning-OFF a switching device that isinitially in its ON-state is shown in FIG. 2. Voltage source 21, wheninterrupted, results in rapid decay of circuit current. Voltage source21 consists of a common Schmitt trigger circuit working off a sine wavegenerator into a one-shot multi-vibrator section and a couplingcapacitor to provide a shaped current pulse as desired. Upon applicationof a first pulse, the switching device 22 is turned-OFF by the rapidlydecaying current, but electrical charge tends to remain on both sides ofthe switch, the side toward the electrical ground as well as the sidetoward the voltage source 21. Such charge, if left unattended, maydevelop sufficient potential difference to turn-ON the switch again.Typical means for removing excess charge promptly is shown in FIG. 2. Itmay bleed through a parallel resistor 23 to ground. At the other side ofthe switch it may bleed through variable series resistor 24 to ground.Further difficulty may still arise because of differences in the timeconstants for bleeding off charge from both sides of the switch. It canbe overcome by those skilled in the art by introducing a pulse timedelay circuit 25, composed of an inductor and capacitors as shown forexample, in series with an appropriate side of the switching device. Aswitch by-pass consisting of a 1N-4005 silicon diode 26 in series with aten millihenry inductor 27, also, serves '-to eliminate transientvoltages in the circuit.

The above circuit is not only useful for turning-OFF a switch, but canbe used for repetitive cycling between ON- and OFF-states. This ispossible because once a switch is turned-OFF it develops a much higherresistance than the internal resistance of voltage source 21. Hence,essentially the full voltage of voltage source 21 can then be made toappear across the switching device in its OFF- state. By voltageregulation the next pulse to be supplied is therefore adjusted in valueto that required to turn-ON the switching device to complete a cyclebetween ON- and OFF-states. Repetitive cyclingrates may be varied fromrelative low frequencies to frequencies of 10,000 Hz. cycles or more andindividual pulse widths from about 1 to 50 microseconds using a typicalSchmitt trigger. Visual observation of alternating ON- and OFF-statesmay be followed by suitable use of an oscilloscope in testing the lifeof switching devices of this invention. Life test show that switchingdevices of this invention can be cycled over 10 times or more withoutfailure using the circuitry of 'FIG. 2. 1

As stated above, the turn-OFF current can be as little as 0.1milliampere. The extended current range makes possible a novel,three-lead, high speed switchingdevice with an isolated second lead.Preferably such a device comprises two bistable switches as describedpreviously in series such that the first switch requires a current toturn-OFF by virtue of its improved composition, particle size, andshape, that is less than either the current to turn-OFF or the currentthat corresponds to the voltage to turn-ON the second switch. The firstbistable switch must be turned-OFF initially. The characteristics of thetwo switches can be selected so that write (ON) and erase (OFF) pulsesfor the second switch pass through the first switch to reach the secondswitch, yet the first switch is always OFF whenever the ON or OFFcondition of the second switch is read, i.e., determined using anisolated second lead between the two switches. Such a lead iselectrically isolated from the write and erase circuits by the highresistance of the first switch when form a 1-2 mil thick film upon airdrying at 40-50 C. for 24 hours. Two electrodes were painted and driedon the film said electrodes being applied as a conductive silverpreparation containing silver powder and being The three-lead switch canbe used, for example, to 5 spaced about 0.5 cm. apart and extendingacross the width store binary information, e.g., as a computer memory ofthe slide. The dope composition with the two applied element b ith relectrodes was then activated by an initial voltage pulse passing anelectrical voltage pulse in series tin-ugh the and performed as a switchas recorded in Table 1 below:

lfirst and second switches, which first switch is turned- ComparativeExample 1-A ON and then turned-igl g'N y i ffg i r gegf g g g dope wasprepared and a switch made as in Example second switch 1s turne an e a lwith the single exception of omitting component (c) of turned -ON by sa1d first pul the said Example 1, and the performance of this switch bypassing an electrical curre P 1!! 861165 T P 1s also recorded in TableI. It will be seen upon examid fi t and second switches, l nation ofTable 1 data that the current to turn-OFF the turned-ON and thenturned-OFF by s p W E ample 1 switch was one-ninth that required totum-OFF second11 swttchdis left turned-OFF, 1.e., turned-OFF Y theswitch of Example l-A.

said P so, an

Exam les 2,3 4 5and6 in a manner consistant with the binary form ofmforma- F I tion to be stored as a conductive condition of the second Wead 9 f were Prepared as 1n Example 1 switch except that in each instancecomponent (a) of Example 1 was replaced by: (2) solutions of chloro reneo1 mer Both the twoand three-lead switches utilizing the com p p ypositions of this invention demonstrate more reliable XykPe (Du PontNeoprene W Synthetlc rubber, 450% characteristics that have advantagesin computer logic g g pg y f s p y 111 tetfahydroand memory systems aswell as in modulation and conu R Adlprene C Polyurethane trol of otherelectrical devices. Other advantages are the 430%. fe block P' of P y ye and simplicity of design and fabrication, particularly the g f' mxylene 1102, 880% py). ease of interconnecting switches for use in highcapacity cls'lATolybutadlene Polymer 1n Xylene (phllllps memories forthe more advanced type of computers or etroleum Rubber], 540% g learningmachines. Switching times are faster than a no), a p y l; of ethyle e,propylene and a microsecond and, "as a result of the low turn-OFFcurrent, lri'hexadlene (Du Pont Nordel 1070 hydrocarbon banks ofswitches of this invention have extremely l ow m 46O' 48O% elongatlon)toluenepower requirements and high packing denslty qp g y- ComparativeExamples 2-A, 3-A, 4-A, 5-A and 6-A These and other applications of thecurrent switc prepared from the compositions of this invention are asgg' j gg gg g zfi l il a i g tg d fave6 switches electronic circuitelements in oscillators, multivibrators, tively except gt in a ch insgglzz comI(:)lZI|1gent ,(esgeg; or flip-flops, relays, circuit breakers,flashers, electromc ommd p displays- EXAMPLES Fromthe results in Table1, it can be seen that inl 40 corporatmg a ferromagnetic component intothe composi- The following examples arg mtendedlto lietrneretllyl 111 1?tlOns according (:0 this invenltsion markedly decreases the trative ofthe invention an not in 1rn1 a ion ereo current requlre to turn-O F theswitches, e.g., the Unless otherwise indicated, all quantities are byweight. smallest effective current to turn-OFF the switch of Ex- 1 ample6, 0.29 ma. proved to be less than one-hundredth EI'KaITIPe 1 that ofthe 6-A switch. Reductions in current when A dope was prepared by mixingat room temperature utilizing compositions of this invention were notedin all and atmospheric pressure (a) 20 cc. of a 15% solution sixcomparisons.

TABLE l.-SWITCH PERFORMANCE Current limited pulse to Resistance Turn-OFFActl- Voltage of the swich vating used to ON-State milli- Usingcomposition ofvoltage Turn-ON (ohms) amperes Examplel 150 19.000 1. 88gompamtive Example 1-A 5%, x m e goi rip fagive Example 2A 132 11 t ifigr t ii "Iiiriirhie'-AII .1 s 1 53 iii i ve'eanaan" 538 1 afi'gg ta efitrfiaa'srtasi B I 150 100 6 22'. 7 Example 6 150 180, 000 0. 29Comparative Example 6A 140 95 500 36. 4

Resistance of each observed OFF-state was measured with a SimpsonVolt-Ohmyst. and was found to be at least 10 ohms.

by weight of an elastomeric (250290% elongation) copolymer of vinylidenefluoride and hexafluoropropylene (Du Pont Viton A Fluoroelastomer) ll'lacetone, (b) 3 grams of nonleafing aluminum powder of average particlesize 19.6 microns, by weight passing through a 325 mesh sieve (AlcanAluminum Co. MD 2000 aluminum powder), and (c) 3 grams of iron powder ofaverage particle size about 20 microns (Baker and Adamson 1807 iron). Asufficient quantity of the dope was deposited on a 1-inch x 3-inch glassmicroscope slide to Example 7 Three dopes were prepared by mixing atroom temperature and atmospheric pressure (a) 20 cc. of a 15 solution byweight of natural rubber (500% elongation) in xylene, (b) 3 grams ofnon-leafing aluminum powder of average particle size 19.6 microns, 85 byweight passing through a 325 mesh sieve (Alcan Aluminum Co. MD 2000aluminum powder), and (c) 1, 3, and 6 grams, respectively, of 200 meshcobalt powder (City Chemical Co.). Switches were prepared and testedusing these three 11 dopes as in Example 1 and their performances areshown by data of Table 2.

Example 8 Switches were prepared and tested as in Example 7 except thatcomponent (c) was 100 mesh cobalt powder (City Chemical Co.).Preformances are recorded by data of Table 2.

Example 9 Switches were prepared as in Example 7 except that component(c) was iron powder of average particle size about 20 microns (Baker andAdamson 1807 iron). Performances are recorded by data of Table 2.

Example 10 Switches were prepared as in Example 7 except that component(c) was 200 mesh iron powder (City Chemical Co.). Performances are shownby data of Table 2.

Example 11 TABLE 2.-SWITCH PERFORMANCE The life of an activated switchprepared from dope composition 6 was tested using the circuit-shown inFIG. 2. Voltage source 21 consisted of a Schmitt trigger delivering 200volt DC pulses 1.0 microsecond in duration at a repetitive cycling rateof 1,000 Hz through'a 0.0003 ,uf. coupling capacitor. The-pulse timedelay circuit 5 consisted of a 10 millihenry choke and ganged f.capacitors. The parallel chargebleed resistor 23 was about 6000 ohms andthe series bleed'resistor 4 was 3500 ohms. The switch was cycledrepetitively for3 hours or 10.8 million cycles. The switch was stillrunning anddid not fall short or b O v 'j .1.: l

TABLE 3 M, Current limited Resist-, pulse to ance Turn-OFF Aeti- Voltageof the switch,

Using Example 11 vating used to n ON-State millicomposition voltageTurn-ON 4 (ohms) vemperes Current limited pulse to Ferro- Resist-Turn-OFF magnetic Acti- Voltage ance of the switch, Using composition 0!powder vating used to 0 ate Example- (grams) voltage Turn-0N ohms)amperes Resistance of each observed OFF-state was measured with 5Simpson Volt-Ohmyst Percent Weight Dope (weight) ratio compositionAl-l-Fe e:Al

1 a Al/LOFe 57 1:3 2. 6 A1 1.0 Fe 70 1:6 3. 1.5 [0.5 Fe 40 1:3

' 4- 3Al/0.5 Fe 54 1:6 5. 3 iii/2.0 Fe 63 2:3

- c A1/3-0 Fe 61 1.1 7 3 Al/6.0 Fe- 75 2:1

Y Switching performance of the switches utilizing the var? ious dopecompositions are recorded in Table 3 which shows that switches made fromcompositions with weight ratios of iron to aluminum within the scope ofthis invention all require less than 5 milliamperes tum-OFF current.

Example 13 A mixture of (a) 100 grams of a terpolymer of ethylene,propylene and a 1,4-hexadiene (Du Pont Nordel 1070 in hydrocarbonrubber, 460-480% elongation) with 7.0 gm. dicumylperoxide added ascuring agent, (b) 100 grams of nonleafing aluminum powder of averageparticle size 19.6 microns, by weight passing through a 325 mesh sieve(Alcan Aluminum Co. MD 2000 aluminum powder), and (c) grams of ironpowder of average particle size about 20 microns (Baker and Adamson 1807iron) was milled with 20 grams of milling oil (Sun Par Oil). The millingoil was then extracted with a mixed solvent composed of equal weights ofperclene and ethanol. The mill-blended mixture wasthen heat-cured at C.for 30 minutes to 'form a 10 mil thick film about 12 inches in diameter.Two conductive silver paint elec' trodes were applied to the surface bybrushing, and a volt age pulse of .400 volts was applied between theelectrodes to form an activated switch of 200 ohmsre'sistance in itsQN-state. The switch was operated successfully between OFF- and'ON-states by'alternate applications of ac1ir' rent pulse 'of20'milliamperesanda voltage pulse pf 200 volts 13 Example 14 Athree-lead switch with an isolated second lead was built by placing theswitch of Example 6 in ser es W1th the switch of Example 3. The latterswitch requires 4.3 ma. to turn OFF and 65 volts to turn ON, so that theformer switch which requires only 0.29 ma. (from a 35- 40 volt source)to turn OFF, remains OFF as the hlgh current switch is pulsed throughthe low current switch to either an ON or an OFF state. This keeps thepulse input terminal isolated so it cannot be read itself yet the stateof the high current switch can always be read directly by a simpleresistance measuring circuit placed across its terminals.

To demonstrate this a voltage pulse or write signal of 0.5 ma. derivedfrom a voltage source of over 65 volts was applied through the lowcurrent switch of Example 6 to the high current switch of Example 3. ASimpson Volt- Ohmyst resistance meter was then connected in series witha blocking resistor of 100,000 ohms resistance and Comparative Example15 Equal weights of Alcan Aluminum Co. aluminum powder MD 2100 andGeneral Aniline and Films PQ-19 fine carbonyl iron powder were mixedtogether, fired in an argon-filled furnace at 1,200" C. until alloyed,and pulverized to pass a 325 mesh screen.

The procedure of Example 1 was repeated twice except that 4.5 grams and7.5 grams, respectively, of the prepared aluminum/iron alloy powder wereused instead of 3 grams each of separate aluminum and iron powdercomponents (b) and (c) of Example 1, and polyurethane cpolymer intetrahydroffuran (Du Pont Adiprene C polyurethane ru=bber 430%elongation) was substituted for component (a) of Example 1.

Two latent switches were prepared from these compositions as in Example1 with two applied electrodes. They could not be activated to a state oflow resistance by an initial voltage pulse in the range of 150 to 400volts and did not perform as switches.

The foregoing detailed description has been given for clarity ofunderstanding only and no unnecessary limitations are to be understoodtherefrom. The invention is not limited to exact details shown anddescribed for obvious modifications will occur to one skilled in theart.

The embodiments of the invention in which an excluiive property orprivilege is claimed are defined as folows:

1. A composition of matter comprising:

(a) aluminum particles,

(b) ferromagnetic particles, and

(c) an elastic binder for (a) and (b), wherein the ratio offerromagnetic to aluminum particles by weight is in the range of 1:6 toabout 2:1 and the combined weights of the aluminum particles and theferromagnetic particles is from about 40 to about 85 percent of thetotal weight of said particles and the elastic binder.

2. A composition according to claim 1 wherein the ferromagneticparticles are selected from the group consisting of iron powders, cobaltpowders, nickel powders and mixtures thereof.

3. A composition according to claim 2 wherein the ferromagneticparticles are iron powders.

4. A composition according to claim 2 wherein the ferromagneticparticles are cobalt powders.

5. A composition according to claim 2 wherein the ferromagneticparticles are nickel powders.

6. A composition according to claim 1 wherein the elastic binder iscapable of being elongated at least 100 percent and still retract toless than 1.5 times its original length.

7. A composition according to claim 6 wherein the elastic binder isselected from the group consisting of natural rubber, syntheticpolyisoprene rubber, elastomeric chloroprene polymers, fluoroolefinelastomers, butadienestyrene rubber, ethylene-propylene-non-conjugateddiene rubbers, silicone rubbers, and polyurethane rubbers.

8. A composition according to claim 6 wherein the elastic binder is in acarrier solvent.

9. A composition according to claim 1 wherein the ratio of ferromagneticto aluminum particles by weight is about 1.521.

10. A composition according to claim 1 wherein the combined weights ofthe aluminum particles and the ferromagnetic particles is from 60 topercent of the total weight of said particles and the elastic binder.

11. A composition according to claim 1 consisting essentially of (a)aluminum particles,

(b) iron particles, and

(c) terpolymer of ethylene, propylene and an unconjugated diene.

12. A composition according to claim 1 sheet form.

13. A composition according to claim 11 which is in sheet form.

14. A composition according to claim 13 wherein the composition has beenhot-pressed into sheets.

15. A composition according to claim 1 which has been activated by theapplication thereto of a voltage pulse to form a conductive path of lessthan one megohm.

16. An activated composition according to claim 15 which has tworesistance states and which is capable of being switched between thesetwo resistance states.

which is in References Cited UNITED STATES PATENTS 3,571,777 3/1971Tully 33820 3,685,028 8/ 1972 Wakabayashi et a1. '33820 X 3,562,1872/1971 Abdella 252-513 OTHER REFERENCES Films Solid-State Electrons,Pergamon Press, vol. 7, pp. 785-797 (1964).

CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R. 25262.54, 512;338-20, 21; 340173.2

